SpudFiles

If it's not sealing with a spherical projectile, I doubt a cylinder is going to help.

One of the issues is that you need to depressurise the fill rig rapidly in order to shock the projectile into sealing (almost like an exhaust valve). I'm using a schrader valve to do this, and the fill rig is 26.5mm inner diameter, more than double the cartridge, so there's a lot of dead space.

I could scale up to 3/8" calibre and probably work better but the Sherline is a miniature lathe

One thing I discovered making this is (as LeMaudit had already pointed out, but you have to feel it to understand) is that it was significantly easier to cut the M10 thread for this one than the M12 for the pen gun, so making a small threaded burst disk holder shouldn't be a problem. It adds to the loading process but it should be more reliable.

Still, the "valveless" principle is too good to let go of without a fight. One thing I thought about was threading the outside of the cartridge in order to connect it directly to a schrader adapter, but it would be easier to just make a smaller fill rig directly.

Another option could be to have the projectile already placed in the seal, and with a separate fill valve in the back (that would also allow you to poke out any projectiles which failed for some reason...)

Yes, I have a lot of brass to go through

Another option could be to have the projectile already placed in the seal, and with a separate fill valve in the back (that would also allow you to poke out any projectiles which failed for some reason...)

One one hand that sounds nice, but on the other... ohh damn that's going to increase mass and complexity

Have you considered creating a screw in plug for the cartridge ? either the pellet holder or the back of the cartridge could be screwed in in place after filling the cartridge

I see what you mean, but now you've added another seal between the two cartridge halves.

I like the idea of simplifying the structure though:

but now you've added another seal between the two cartridge halves.

o-rings are quite cheap you know

and that solves all the problems that adding a shrader valve won't solve

POLAND_SPUD wrote:...and that solves all the problems that adding a shrader valve won't solve

For the extra complication though you could still fill with the rig, but add a simple "drawing pin" valve at the back. I wasn't proposing adding a schrader, trying to avoid those for cartridges

I drilled out the back of the cartridge and added a simple check valve in order to try it out as a "conventional" burst disk cartridge. Epoxy curing, testing this evening hopefully.

a "conventional" burst disk cartridge

A pneumatic cartridge ?? you diagram suggests that there is a metal insert in the epoxy plug so...

POLAND_SPUD wrote:A pneumatic cartridge ?? you diagram suggests that there is a metal insert in the epoxy plug so...

Still a hybrid, the intention is to use the fill rig and have the igniter contacts with the valve and cartridge body.

Edit: spark gap works well, now to see if it seals. Watch this space

Jsr, why not use a piston? It may work a little bit better, and if you get spring strength right, then you wont even need a pilot valve for the piston, the main combustion of the hybrid could smash the piston back wards opening up the barrel allowing the exhaust to flow out. I would try it if I had some syringe heads.

warhead052 wrote:Jsr, why not use a piston? It may work a little bit better, and if you get spring strength right, then you wont even need a pilot valve for the piston, the main combustion of the hybrid could smash the piston back wards opening up the barrel allowing the exhaust to flow out.

Simpsons did it.

While it's more practical than a burst disk, there are some disadvantages, namely if it's a coaxial the cartridge is much more bulky, and it complicates ignition.

In other news, tried out the modded cartridge with air at 75psi, held pressure beautifully The epoxy bulged out a little though, indicating it's not cured yet so I put it aside. Still, looks promising.

Update after some frustrating testing, tried it at 6x - it held the pressure well enough, but the spark was arcing externally, clearly the spark gap inside was not small enough. It needs to be put together with more precision.

How is it going?

Did you consider machining the plug in some gluable plastic (Delrin?) or simply use a plastic sleeve and a rubber washer to insulate the central part?

LeMaudit wrote:How is it going?

Right now, it isn't... well, not very fast at least.

Did you consider machining the plug in some gluable plastic (Delrin?)

I had started to make an acrylic plug but it shattered when I was trying to thread it... got some teflon rod cutoffs from work today, but they have a 10mm hole through the centre... but yes, when I source some suitable material, it will be a perfect idea.

or simply use a plastic sleeve and a rubber washer to insulate the central part?

Since we like making graphic tutorials, my turn the valve stem could be shortened, but then you would also have to suitably insulate the end of the electrode which goes in to make contact with it.

So what's to stop you from insulating the valve stem? Or using a plastic one?

Oh! I see... I didn't catch the pressure had an influence... of course it makes sense...

Alright, here's a better insulated-no thread-no glued-self sealing one